It is often desirable to have the ability to measure changes in materials as they undergo various chemical and biological reactions in real time. Direct detection is one method employed for such measurements and is typically based on optical techniques that respond to changes in refractive index. The optical response has been attributed strictly to mass capture as molecules are bound to the sensor surface.
Hsieh et al. (U.S. Pat. No. 6,576,430 B1) describe a method for detecting and analyzing allosteric receptor/ligand binding by monitoring changes in the surface refractive index. They found that changes in refractive index can be used to detect binding of ligands to immobilized receptors when the receptor (e.g., a binding protein) or the receptor-surface complex undergoes a conformational change upon binding to the ligand. The conformational change is detectable even when the ligand is small and the receptor is large. By employing Surface Plasmon Resonance (SPR) techniques, they found that binding of such allosteric binding agents to their ligands may produce negative deviations in the optical response or a decrease in resonance angle. Alternatively, they also found that an increase in refractive index was observed for allosteric binding agents which “open” upon binding of their ligands, resulting in a positive deviation in the optical response. By observing these deviations, they are able to detect receptor/ligand binding even when the ligand is small. When employing similar techniques in long period grating (LPG) systems, they observed an opposite result from that of SPR. They concluded that changes observed on LPG do not always follow those observed on SPR. This method is limited to single sensing depths because only one polarization state can transfer energy into the metallic layer of the SPR sensor. In the case of a LPG system, only one binding depth could be detected and there was no way to distinguish between different parameters. Thus, when there are changes in two parameters occurring simultaneously, such as protein shape (density) and mass capture, these changes cannot be separately distinguished.
Murphy et al. (U.S. Pat. No. 5,864,641) disclose an optical fiber long period sensor having a reactive coating. The reactive coating is positioned in an operable relationship to the long period grating and causes the long period grating to produce a wavelength transmission spectrum functionally dependent on a parameter sensed. When the chemically reactive coating has target sites present, a chemical bond is formed between the target site and the specific molecule. This causes a change in the wavelength transmission spectrum produced by the long period grating. The coatings and the positional relationship of the coating with respect to the long period grating sensor described by Murphy et al. are suitable for use in practice of the present invention.
Pitner et al. (U.S. Patent Application Publication 2003/0130167 A1) modified the sensor of Murphy et al. such that the affinity coating contained mutated proteins capable of detecting binding between analyte and mutated Galactose/Glucose Binding Protein. The modified sensor was used to measure refractive index changes as binding occurred. However, such changes were still limited to single parameter measurements.
An object of the present invention is to provide a fiber-optic method for making simultaneous multiple parameter measurements using an optical fiber having at least one long period grating.
Another object of the present invention is to provide a way to identify changes in a reactive coating as it reacts with a material by correlating shifts in at least two different long period grating signatures with changes in the material and solving a series of equations.
Another object of the present invention is to provide a method for identifying changes in a material as it is applied to a surface of an optical fiber sensor by comparing shifts in at least two long period grating signatures, correlating the shifts to changes in the material, and solving a series of equations.